Teletypewriter trunk circuit



2 Sheets-Sheet l Filed June 9, 1951 /Nl/ENTOR J. R, DA VE Y Jeni... E.Cm?

ATTORNEY April 27, 1954 J. R. DAVEY TELETYPEWRITER TRUNK CIRCUIT FiledJune 9, 1951 FIG. 2

INDIVIDUAL 7'E LE TYPE WRITER 5 U85 C RIEE R 7A TIONJ` REMOTE FROM TELETYPE WRITER CENTRAL SWITCH/NG OF FICE 5 TRUNK CNDUCTO/` 0F PRESENTSPEC/AL TES T/NG TRUNK 2 Sheets-Sheet 2 PRIMA R Y TELE TYPE WRITER CENTRAL `W/TCH/NG OFF ICE SECONDARY TELETYPE WRI TER CENTRAL `SWITCH/NGOFF/CE *f CROSSBA R SWITCHES TELETYPEWR/TER N SUBSCRIBE? TOLL LINECIRCUITS 5A TELL /TE EXCHANGE OR CONCENTRATING UNIT RELA Y EQUIPMENT 0FPRESENT SPL-cmi. 205

TEST/NG TRU/wf JACK APPEARANCES LAMP TELEGRAPH SERV/CE BOARD OFPRESENTSPECIAL TEST/NG TRUNK TEST/NG CIRCUIT PLUGS TELEGRAPH SERV/CEBOARD OFF CE /M/ENTOP J. R. DA VE V j@ CM A TTORNEV Patented Apr. 27,1954 TELETYPEWRITER rRUNK CIRCUIT James R. Davey, Franklin Township,Somerset County, N. J., assigner to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationJune 9, 1951, Serial No. 230,755

Claims. (Cl. 178--73) This invention relates to telegraph systems andmore particularly to a new type telegraph trunk circuit one end of whichis arranged to operate on a potential basis, so that it may be connectedto circuits which operate on a potential basis, and the other end ofwhich is arranged tc operate on a current basis, so that it may beconnected to circuits which operate on ay current basis. The end of thetrunk which operates on a potential basis is arranged, in the presentembodiment for termination in a telegraph service board which board hastelegraph facilities arranged for telegraph hub repeater transmissionand employing electronic controls therefor, which controls operate on apotential basis. Such facilities are. well known in the art beingdescribed for instance in Patent 2,542,208, granted to M. R. Purvis,February 2G, 1951, which is hereby made part of the. present disclosureas though fully set forth herein.

An object ofthe invention is the improvement of telegraph systems. Amore particular object o the invention is the provision of a telegraphtrunk circuit arranged for cooperation with teletypewriter or telegraphcord circuits, monitoring oord circuits and other cord circuits whichfunction on a potential rather than on a current basis.

vThe testing and maintenance facilities of telegraph switching centerswhich have telegraph lines and telegraph loops arranged for useV intelegraph concentrations on ahub basis, are provided with varioustesting and communication cords specially arranged for cooperation withhub type telegraph repeaters. The scrviceraforded by such facilities isnow being extended so thatv they may be employed for testing telegraphcir- .k concentrating units or satellite exchanges as theyV aresoinetnnes'callcd and' such trunks are well shown in the art'. They are`described forinstance Patent 2,228,279, granted' to C. W.

T le, i941; Patentraeea grant- 1 Luces. c

ed to? i nnreaddlsucek. February l, 194i;Y and Patent 2,368,666, grantedto J. A. Krecek, February 6, 1945, which are hereby made part of thepresent disclosure as though fully set forth herein. Such trunks are notequipped with hub type repeaters. It is desirable to arrange them sothat they may be maintained and tested With the various cords employedin a telegraph service board which cords are designed to function withhub circuits.

In the present invention, as an example of the solution of this general'problem, a. special testing trunk circuit is employed which extends froma concentrating unit to a test position at a telegraph service board.The special trunk is arranged so that at one end it may be connected atthe concentrating unit to the masterv signaling trunk which is to betested, which master trunk operates on a current basis, and at the otherend it may be connected, in the testing position of a hub type telegraphservice board, tor the various cord circuits employed in testing whichare intended primarily to function with hub repeater circuits havinghubs which operate on a potential rather than on a current basis.

The present trunk is arranged therefore to operate through one end on acurrent basis and through the other end on a. potential basis.

The master trunk interconnecting a concentration unit or satellite oiceand a master office which is extended by means of the present trunk tothe hub type testing facilities, itis emphasized, is cited only by wayof example and it is to be understood that the present invention willalord means whereby telegraph circuits which operate on a current ratherthan on a potential basis may be tested by testing circuits whichoperate on a potential basis.

The invention may Vbe understood from the following description whenread with reference to the associated drawings. it is to be understoodthat the invention may be incorporated in other embodiments than that inwhich it is presently shown which is a preferred embodimentv of the Yinvention.

an automatic; signaling trunk to a test board for testing andcommunication for maintenance purposes. It is emphasized that theinvention consists in a trunk circuit having one end arranged to operateon a potential basis and the other end arranged to operate on a currentbasis. The invention may be applied in any service where it is desirableto interconnect a circuit which operates on a potential basis with onewhich operates on a current basis and a trunk is required tointerconnect the two. In the present embodiment of the special trunk,the trunk serves as a link between testing and communication circuits ata test position in a telegraph service board and another trunk which isto be tested. The trunk which is to be tested is a trunk which isemployed to interconnect a concentratingr unit known also as a satelliteoffice and a teletypewriter switching central ofce.

In Fig. 2, the rectangle EBI represents the telegraph service boardcnice in which there is a telegraph service board 292, having a testposition 2%. At the test position there are testing circuits which maybe terminated in plugs such as Zilli. Up to the present time all of thecircuits which are tested at the service board operate on a potentialbasis and the testing circuits such as Zii are designed to cooperatewith such circuits.

A satellite exhange or concentrating unit, if it is to be feasibleeconomically, is generally unattended. It is desirable to be able totest the facilities which terminate at the satellite exchange from aremote testing position to save sending a test man regularly to thesatellite exchange. The difficulty in the Way of testing the automatictrunks, which connect the satellite exchange to the centralteletypewriter switching oice, with the testing facilities in the testposi tion at the telegraph service board is that the testing facilitiesat the service board are arranged to test circuits which operate on apotential basis, whereas the automatic trunks operate on a currentbasis. This dimculty is obviated by the present invention which aords atrunk having the service board end arranged to cooperate with testingand communication circuits which function on a potential basis and theremote end which connects to the automatic trunks at the satelliteexhange arranged to interconnect with circuits which function on acurrent basis.

The jacks of the present trunk designated in Fig. 1 as PRI CO and SEC COand the lamp ANS appear at the test position 283 and are indicated as agroup 2te. They are shown connected to the trunk relay equipment 265which is connected to the satellite exchange or concentrating unit bymeans of a group of conductors indicated by ccnductor 206.

As usually arranged a number of teletypewriter subscriber stations 2li?remote from the nearest teletypewriter` central switching omce areconnected to a satellite exchange 238, where each terminates inindividual relay and other equipment 209 called a subscriber linecircuit, or a subscriber toll line circuit when the distance from thesatellite office to the subscriber station is long, which is frequentlythe case. At the satellite exchange are crossbar switches 2i@ whichserve to interconnect the subscriber lines and the automatic mastersignaling trunks 2li which extend to the primary and secondaryteletypewriter central switching office ZIE and 2lb, respectively.

At the satellite exchange, one oi the subscriber line circuits 263,which as explained constitutes relay and other equipment, identical withthat employed at the satellite exchange to connect a subscriber stationto a crossbar switch which extends the path to an automatic masternaling trunk, is employed to connect the special trunk of the presentinvention to a crossbar switch and to the automatic master signoiptrunk. The rectangle in Fig. 1 designated circuit at concentratorequipment compri jacks and wiring which afford a test man accese; to thepresent trunk for testing the present trunk from the satellite exchangeon such occasions it may be necessary for a test man to visit thesatellite office. This equipment may be considered as part of thesubscriber line circuit. corresponds to jacks 610 and 611 and theassociated wiring shown in Fig. 6 of Patent All of the equipmentindicated in 2 herein is Well known in the art except the present spec'testing trunk, comprising the jacks and Mit, the relay equipment ZS andthe conductors 20G. The known equipment is described in detail in thefour patents listed in the foregoing.

To amplify the foregoing somewhat, as an aid in understanding therelationship of the present special testing trunk of the invention tothe circuits, known in the art, with which it cooperates, in theoperation of telegraph satellite systems, or concentrating systems, anumber of teletypewriter switching subscribers too remote from a centralteletypewriter switching exchange to be connected thereto economically,because of the original and maintenance expense of the long circuitswhich would be required, are connected to a small automatic switchingcenter and a relatively small group of trunks interconnecting theautomatic switching center to the central teletypewriter switchingexchange are employed to provide sern vice for a larger number ofsubscribers. For instance twenty teletypewriter subscribers to the eastof Garden City, New York, instead oi being connected individually, eachby a separate circuit, to a teletypewriter switching exchange in NewYork may be connected to a small automatic ex change in Garden Citywhich in turn will be nected by means of a small group, such as iiautomatic trunks, called master signaling trun to New York. Theautomatic Garden City oiiice is called a satellite ofce or concentratingu1 Such satellite installations are generally fully automatic. To befeasible it is necessary the equipment in the Garden City exchange be teregularly to detect and preferably to anticipate trouble. 1n order to dothis, testing will be provided through a testing trunkA which connects ta test position at a telegraph service board nearby telegraph switchingoffice. In one l r e modern telegraph system the test board is quired totest large numbers of circuits which systems. In many telegraph offices,private wire systems, as distinguished from teletypewriter switchingsystems, are interconnected through hubs and in the more recentlydeveloped hub circuits the operation is on a potential rather than on acurrent basis. In many modern telegraph offices all of the telegraphlines are connected through a telegraph service board. They are equippedwith so-called hub repeatercontrol circuits more recently termedcoupling units. thus arranged any telegraph line coming into telegraphoice may, if desired, be connected to a group of othel1 linesI similarlyequipped 'through a telegraph hub into a private line network to servesay a particular single corporation. The modern hubs operate on apotential basis. En

order that telegraph lines. vor telegraph loops arranged forinterconnection through hubs which operate` on a potential basis maybetested, it. is necessaryl to provide special. communication testingcircuits which operate also on a` potential basis. The automatic trunksinterconnecting a concentration unit to a teletypewriter centralswitching station, as mentioned,y operateon a current basis. If theadjacent testing facility, to which they are to be connected for testingmaintenance, operates. on a potential basis, it is required that somespecial means be provided so that the current trunks be testable withthe potential testing circuits.

In the present. invention a special trunk; is provided. It is connectedfrom ay test position at4 an electronic. hub type service board' whichoperates on a potential basis to the satellite or concentrating unit asit is called. It is arranged so that at its end which terminates in thesatellite exchange it is connectable to lany or the ve trunks mentionedin the foregoing which are lznown in the art as master signaling trunks,and which, as mentioned, extend from the satellite apparatus to thecentral teletypewriter exchange. The trunk to the central exchangeoperates on a current basis. The test position circuits operateV on a`potential basis. The trunk operate on the portion connected directly tothe satellite end on a current basis and on the portion connecteddirectly to the service board end on a potential basis.

The satellite 'Switching apparatus as mentioned is mechanical. Thepresent trunk incoming to the satellite exchange in certain respects maybe considered to resemble a cir-cuit incoming to the satellite exchangefrom a subscriber teletypewriter station. In normal operation theteletypewriter subscriber dials pulses and in response to this isconnected through the satellite apparatus to the central telctypewriterexchange. The present trunk incoming from the service board testingposition terminates at the satellite exchange in the same manner as doesa subscriber loop. At the service board testing position the trunk ismanually operated. In the present embodiment it terminates at theservi-ce board testing position in a jack. rFlic test man at the serviceboard testingposition connects a com.- inunication or testing circuit tothe jack termination of the present trunk. In response to this thepresent trunk is connected through the mechancal switching apparatus atthe satellite oiice to the automatic signaling master oficetrunk whichentends to the master teletypewriter switching cnice. Y

In modern satellite systems ordinarily there will. be provided trunksfrom the satellitel oice to more than one central teletypewriterexchange so that an alternate route is 'available if. another is busy.In the present trunk, arrangements are afforded whereby connections maybe made over master trunks to more than `one central teletypewriterexchange. v t

The circuitv shown in the drawing, Fig. l, and its relationship tocircuits known in the, art in Fig. 2, provides, at the satelliteapparatus, the equivalent of a. subscriber appearance, which permits theAattendant at the telegraph service board to make testsof theautornaticsignaling master oiiice trunks extending to a teletypewriter switchingexchange. Y y y By means of directing pulses sent ori-tirom the mastercentral tel-etypewriter switching exchange 6 over the automaticysignaling master office trunk to the satellite apparatus, the mastertrunk may be extended through the present trunk to va test position at aservceboard.

First to consider the functionsl generally:

The automatic signaling master oiiice trunk is connected at; thesatellite exchangeV to the present circuit byA way of the. automaticswitching equipnient at the satellite exchange in response to the propercall directing pulses transmitted from the teletypewri-ter exchangeencl.

Ringing signals incoming over the present trunk to the telegraph.service board termination cause the answering lamp ANS' at the serviceboard to light.

A call is answered at the local test position of the service board byconnecting a teletypewriter cord circuit to the primary cerrtal. oiTicejack PRI CO. The answering lamp ANS is thereupon extinguished.

Disconnection rat the distant end cau-ses the service board attend-antsteletypewriter connected to the telety-pewriter cord circuit,v at thetest1V position at the telegraph service board, to run open asindication of. the distant disconnection.

Disconnection of the teleiiypewriter cord circuit from the presenttrunk, at the test. position at the telegraph service board restores thepresent trunk to its. normal unoperated condition.

A. call outgoing from the test position ofthe telegraph service :boardover the present trunk is originated by plugging the `'ack of theteletypewriter cord circuit into jia-ck PRI C'O for calling over thepresent trunk through the satellite oid-ice for connection on anautomatic signaling master oiiiceV trunk to the primary central omce orby plugging into the secondary cen-trai oiiice ia-cl; SEC CO for callingover the present trunk through the satellite ofi-"ice for connection onan automatic signaling master oiiicetrunk tol the alternate centraloiiice switching point.

A recall is made by disconnecting from either trunk jack, at the testposition in the telegraph service board, for two seco-nds and thenreplugging.

' Incoming cull.'

rihe present trunk circuit connects to the automatic equip-ment at. theconcentrating unit or satellite exchange like `a subscriber loop. Callsfor the present trunk come to the concentrating unit over an automaticsignaling master cinco trunk. 1

The master oiice trunk at the master teletypen writer switching oiceextends through a test 'board tliereat before, it terminates at theteletypewriter switchboardV thereatand calls incoming over the presenttrunk. to the service board test position may be originated at, eitherthe switchboard at the master oiiioe or at the test board. thereat.. Itis possible alsor to employ the present. trunk for a local connectionfrom a jack circuit in the concentration iack circuit of the serviceboard.

' seither case proper calldirecting pulses will cause the concentratingunit to connect the automatic signalingA master oilice trunk terminatingAcircuit to this trunk. Ringing current may he d tothe pi-"esentA trunkautomatically or may be started by the originating operator or attendantdepending upon the traffic arrangement. riihe'ringing current incomingthrough the subscriber line circuitV in the concentrating unit passesthrough the tip conductor T through contact of relay K9, resistance R24,contact 3 of relay KI, resistance RZI, condenser C5 and the winding ofrelay K2 to ground, operating relay K2. The operation of relay K2 inturn operates relay K3 over 'a circuit from ground through contact I ofrelay K2 and the winding of relay K3 to battery. Relay K3 operated locksover a circuit from battery through the winding and contact I of relayK3, contact 'I of relay KI, and contact 3 of relay K9 to ground. Theoperation of relay K3 'also establishes a circuit from battery throughcontact 2 of relay K3 and the filament of answering lamp ANS to groundlighting the answering lamp.

Answering the call The call is answered by connecting the teletypewritercord circuit per Fig. 4 of Patent 2,542,208, mentioned in the foregoing,to jack PRI CO. In response to this a circuit is established from groundon the sleeve of the connecting cord through a resistance in the sleeveof the cord, the sleeve of jack PRI CO and the winding of relay K6 tobattery operating relay K6. The operation of relay K6 in turnestablishes a circuit from ground through the Contact of relay K5 andthe winding of relay KI to battery operating relay KI. The operation ofrelay KI establishes a number of circuits as follows:

It closes the tip of the circuit T from the subscriber line circuit inthe concentrating unit, shown for instance in Patent 2,368,666,mentioned in the foregoing, through contact 5 of relay K9, resistanceR24, contact 8 of relay KI, resistance R2 in shunt with capacitance C5,bottom winding of relay K4, resistance R3, armature and marking contactI of relay K5 and contact I oi relay KI to ground. The operation ofrelay KI also establishes a circuit from the ring conductor R of thesubscriber line circuit in the concentrating unit, through contact I ofrelay K9, contact I of relay K1 which when closed shortcircuitsresistance R23, contact 2 of relay KI and resistance R22 to positivebattery to perform a supervisory function. The operation of relay KIalso completes the transmission circuits of relays K4 and K5, which willbecome apparent hereinafter.

The operation of relay KI by opening its contact 1 opens the lockingpath of relay K3 which releases, extinguishing the answering lamp ANS.The operation of relay KI also performs other functions which availnothing at this time.

Transmission is effected between the tip conductor T of the subscriberline circuit and the teletypewriter cord circuit connected to jack PRICO through the conversion repeater consisting or relays K4 and K5 andthe associated resistances and condensers.

Where the magnitudes of constants are cited in the following it is to beunderstood that the cited values are by way of example to facilitate anunderstanding of the invention but the operation of the circuit is notlimited to elements of the cited magnitudes.

Transmission on the concentrating unit side of the repeater is on a 0-20milliampere balanced loop basis and transmission on the service boardside is on a hub basis with positive 60 volts for marking and negative30 volts for spacing. The connected teletypewriter cord circuit receivessignals on the ring RI and sends on the tip TI which conductors aremultipled to both jacks PRI CO and SEC CO.

First to consider the operation of relay K5, which controls thetransmission of signals originating in the connected teletypewriter cordcircuit and from the armature and contacts of which relay the repeatedsignals are transmitted to the concentrating unit.

For the marking condition the tip conductor TI is open in the connectedcord circuit at the anode of a rst or sending space discharge device inthe tip circuit of the connected cord. A circuit may be traced from thetip of the jack PRI CO and SEC CO in parallel through conductor TI toparallel branches. One branch extends through contact 3 or" relay KI tothe lefthand terminal of the top or operating winding of relay K5 andthen through the winding to the junction between resistances RQA and REBin the potentiometer which may be traced from positive battery throughresistances R8, RGB, RSA, RII and contact I of relay KI to ground. Theother parallel branch may be traced through rectiers CRS, CRS and CRI!to the junction between resistors RI3 and RI4 in another potentiometer,the circuit for which may be traced from positive 13G-volt batterythrough resistances Rlli and RI3 to ground. For this condition thepotential at the junction between resistances RSA and RBB is positive 60volts and the potential of the junction between resistances RI3 and RI@is also positive 60 volts so that no current iiows through the top oroperating winding of relay K5 for the marking condition. Relay K5 isequipped with a biasing circuit which may be traced from positivebattery through resistance R5, lower winding of relay K5 and contact Iof relay KI to ground. The current in the biasing winding remainsconstant at 10 milliamperes and its effect tends to actuate the armatureof relay KI constantly toward its marking contact. During the markingcondition of the transmitting side of the connected cord circuittherefore the armature of relay K5 will be maintained in engagement withits left-hand or marking contact I. For a spacing signal from theconnected cord the rst or sending space discharge device in the cord isactivated and a current of 30 milliamperes ows through the device fromthe present trunk. Of this current 20 milliamperes flow from thepotentiometer comprising resistances R3, RSA, RSB and RII through thetop winding of relay K5. The potential of the junction betweenresistances RSA and RBB as a result of this falls to negative 30 volts.The current through the tcp winding of relay K5 is in such a directionthat its eiect tends to activate the armature of relay K5 to engage withits right-hand or spacing contact and this effect is dominant over` thebiasing effect, so that the armature of relay K5 is actuated to engageits spacing contact. As a result of the conduction of the spacedischarge device in the tip circuit of the connecting 'cord throughconductor TI, 10 milliamperes iiow from the junction between resistancesRI3 and Rli through rectifiers CR4, CRB and CRS. The potential of thejunction of resistors RI3 and Rill falls to negative 30 volts for thisspacing condition.

When the armature of relay K5 is in engagement with its marking contact,ground is connected from contact I of relay KI, contact I and armatureof relay K5 and resistor R3 to parallel branches at the apex of relayK4. One branch extends through the bottom or operating winding of relayK4, resistance R2 shunted by capacitance C5, contact 8 of relay KI,resistor "R24, contact 5 of rela-y iKil, conductor 'I' andthe armature.and `marking contact-of the-sending *re-- lay in the 'connectedl'subs'criberfline circuit'in the concentrating unit to positivet13G-volt battery. A current of 20 milliamperes rilows throughlthebottom or operating-winding i relay K4 -for this condition in such adirection that its effect Ytends to actuate its armature 1to1-engageYits left-hand or marking Contact. Aipath may also be traced. from theapex -of relay K4 vthrough `its yupper or biasing winding `to thejunction'o'f 'resistors vRIE and RIS in a potentiometer-circuit whichYmaybe traced from positive battery through 'resistors RIS, RI5 andContact :I of 'relay KI to ground. The open circuit lpotentialofthefjunctionof -resistances Rl e andRIisapositiveG volts. A current ofl0 milliamperes viioxvs in .the biasing circuit in a directionfrom-:left to right 'and its "effect tends to actuate'th'e armature 'ofvrelayKi to engage its right-hand 'or spacing contact. `For the markingcondition the reif'ect of the current in the bottom or operating windingis idominant and the armature of relay K4 is maintained Ain engagementwith its -lei-tehand or marking c'ontact I. When the armature of relayK5 is actuto engage its spacing contact, .positive v130- volt battery isconnected to the apex of relay K4. No current Vflows therefore inthevlbottombr operating winding of relay K6 since both ends of the circuitterminate in :battery of the same Epolarity and magnitude. Current owsisrompositive 130 volts to positive .65 volts in a direction from rightto left through the biasing winding' 'of relay Kai and its effectnowtends to actuate the armature oi relay K4 toward its marking contactand since the eiect is unopposed the armature or" relay Kt will bemaintained in engagement with its marking contact. Thus .signals maybetransmitted from the contacts `of relay K5 to the concentrating unitwhile the armature of relay K4 remains on its marking contact.

The potentiometer formed by resistors RIS and RIZ: has a potential ofpositive 60 volts for the marking condition which is impressed throughthe ring conductor RI on the ring of jacks PRI CO and SEC CO` inparallel to produce a marking condition in the connected cord. Thepotentiometer formed by resistors R8, RBB, RSA and RI! produces apositive 60-volt potential on the tip Ti of the connected cord formarking.

-When a spacing signal is transmitted from the connected cord asexplained previously l0 milliarnperes fiow from the junction ofresistances Ri?, and RIA. This changes the potential of the junctionbetween these resistances from positive 60 volts to negative 80 volts.The ring circuit RI oi the jacks PRI CO or SEC CO is connected to thegrid of a second or receiving space discharge device in the ring circuitoi the connected cord. This device is maintained conducting for thepositive GU-volt marking potential and is cut off for the negativeSil-'volt spacing potential at the junction between resistances RI3 andRM. In response to this a teletypewriter receiving .magnet the output ofthe receiving space discharge device connected to the receiving or ringcircuit of the cord receives a home copy of the signals transmitted onthe sending side of the connected cord, the signals being transmittedback from the potentiometer in the conversion repeater of the trunkcircuit. This is an important feature of the invention.

A spacing signal from the concentrator results from the connection ofthe armatureof the sending relay in the concentrator to ground which is1'0 impressedon `the left-hand 'terminal of the bottom or operatingwinding vof relay K4. Since ground is connected also in the trunk to theopposite right-handt'erm'inal ofthe winding through -contact iof relayKI, there is Yno'current lin the operating 'winding of relay K13. Theeffect 'of the current in the rupper or biasing winding of relay Ke'which is 'tending to actuate its armature to "its 'spacing "contact'will be unchanged and 'since 'it Ais Vnew unopposed it will beeffective 'to actuate the armature of relay K4 to its righthand orspacing Contact.

circuit may be traced from 'negative 130- volt lbattery throughresistors Rill vand RIZ to ground. For the marking condition of 'relayK4 a `circuit may ibe traced from positive 13G-volt 'battery through thearmature and left-hand 'or marking 'contact I of relay K4 and resistorR5 to the junction 4of resistors RII) and R12. This junction isconnected through rectiers CRi, CB2 and CRS to the junction betweenresistors RIB and Rid. vFor the marking condition the junction betweenresistors R5, Ri and RIZ is posi- Vtive 65 volts and the junctionbetween resistors Rid and RI3 is 'positive '60 volts. Non-linearresistors CRA, 'CR and CRB are connected so 'that or this condi-tiontheir resistance is very high. For the spacing Ycondition of relay Kilwith negative volts connected through its right-hand contact 2 to itsarmature, however, the potential of the junction of resistors RIB and RI2 becomes negative. In response to this a lO-millia'rnpere current rlowsthrough non-linear resistors CRE, CE2 and CR3 and the potential of thejunction ci resistors Rit and RM falls to negative 3G volts. Thisrepeats the incoming spacing signal over the ring conductor Ri into thering of the cord. While conductor Ri is at the spacing value of 30 voltsconductor TI remains at the marking value of +66 'volts and resistorsCRII., CR5 and Citt are high resistance thus isolating conductors Rl andTi. Thus current-no-'current signals 'are transmitted from theconcentration unit and reproduced as potential signals on the receivingend of the trunk.

Disconnection at the distant end Disconnection at the distant end or atthe intermediate test point will cause a spacing signal to be receivedby the trunk which when transmitted through the connected cord willcause the attendants teletypewriter to run open.

Disconne'ctz'on of cord from trunk Disconnection of the cord from thetrunk opens the path through the winding of relay KS releasing the relaywhich in turn releases relay KE. This in turn, by opening contact 2 ofrelay Ki, opens the ring circuit into. the concentrator unit. By thetransfer of the bottom armature oi relay KIA rvfrom contact S to ContactSi, the tip conductor -lrom the concentrator is transferred from thepath through the operating winding or relay K4 to the path through theringing relay K2 in 'preparation for the nekt call incoming to theservice board over the present trunk. The release o-'f relay KI alsocloses a possible locking ground -for relay K3 through contact I ofrelay K3 andvco-n'tact 'i of relay KI. The release vof relay Ki byope-ning its contact I opens the ground connection vfor' relays Kili andK5 and otherwise completely restores the circuit to normal.

A disconnect indication will be sent out from 11 the concentrating uniton the automatic signaling master oice trunk.

Outgoing call A call may be originated by plugging the teletypewritercord into the primary central oiice jack PRI CO for calling on anautomatic signaling master oiice trunk to the primary central office orby plugging into the secondary central oflice trunk SEC CO fortransmitting a call over an automatic signaling master omce trunk to thesecondary central oice switching point. Selection of the central oce isdetermined by the conditions applied to the ring conductor R of thesubscriber line circuit to the concentrating unit to which the presenttrunk is connected. When the primary olice is to be selected positive13o-volt battery through resistance R22 is connected to ring conductor Rby the operation of relays K6 and Kl in sequence. When the secondarycentral oce is to be selected, a momentary high resistance conditionproduced by resistances R22 and R23 in series is connected to the ringconductor R in response to the operation of relay KS which responds tothe connection of battery to the sleeve of jack SEC CO- and in turnoperates relay K1 over a circuit from ground through the contact ofrelay K3, contact 5 of relay KI and the winding of relay Kl. Relay K1 inturn operates relay KI l'the operation of relay Ki establishes a circuitfrom battery through resistances R22 and R23 in series and throughcontact I or" relay K9 to conductor R which extends over the subscriberline to the concentration unit. Relay KT does not remain operated, butreleases as soon after relay Ki operates as the slow-to-releasecharacteristic of relay K1 will permit. Battery through resistances R22and R23 will be applied to the ring conductor R only during thereleasing time oi' relay K1. Relay KI locks operated through its owncontact 4 to ground on relay K8. Other contacts of relay Ki conditionthe tip lead for transmission. In order to insure that the outgoing callwill be over a particular automatic signaling master olice trunkcircuit, it is necessary to manipulate the make-busy feature oi allother such trunks in the service board at the time a call for aparticular trunk is made.

Recall A recall indication may be sent over this trunk to the automaticsignaling master oiiice trunk by removing the teletypewriter cord fromthe jack which is being used in the present trunk for an interval ofapproximately 2 seconds. The relays in the present trunk function in thesame manner as for a disconnect and then as for the origination of a newcall.

Use at concentrating unit When the subscriber iine circuit to which thistrunk circuit is connected must also be used with the test circuitassociated with the line concentrating unit the key designated TB-CO,the ground lamp TB-GD and relay K9 are provided and connected as shown.The key is operated while the trunk is being used at the test positionof the concentrating unit and should be released when the circuit is notin use.

The TB-GD lamp is a guard to indicate the use of the circuit at theservice board or the operation of the TB-CO key. The operation of thekey operates relay K9 and disconnects the tip and ring conductors T andR extending to the concentrating unit from the present trunk.

What is claimed is:

l. In a telegraph system, a trunk circuit, a polar transmitting relay insaid trunk circuit, batteries connected to contacts on said relayresponsive to first signals received by said trunk for transmittingcurrent pulses as iirst telegraph signaling conditions to a firstcircuit connected on one side of said trunk, means including apotentiometer in said trunk responsive to second signals received bysaid trunk for changing the potential of a second circuit connected onthe other side of said trunk substantially to repeat said second signalsto said second circuit.

2. In a telegraph system, a trunk circuit having a iirst potentiometer,a second potentiometer, a relay winding, a rectifier, a circuit pathextending from a rst point in said iirst potentiometer through saidwinding and said rectier to a second point in said second potentiometerand switching means for changing the potential of the junction betweensaid winding and said rectier to control the W of current through saidwinding.

3. In a telegraph system, a trunk circuit, a polar relay therein, aWinding on said relay, a rst potentiometer, a dry rectifier, a secondpotentiometer, a rst circuit path extending from a rst point on said rstpotentiometer through said Winding and said rectifier to a second pointon said second potentiometer, a second circuit path extending from athird point between said rectifier and said winding to a switching meansconnectable to a connecting circuit and a third circuit extending fromsaid second point to a switching means connectable to a connectingcircuit.

4. In a telegraph system, a trunk circuit having a rst and a second end,iirst means in said trunk circuit, comprising a potentiometer controlledsignaling device, for receiving changes in potential conditions throughsaid rst end as telegraph signal impulses, second means in said circuitresponsive to said changes in potential conditions for transmittingchanges in current conditions through said second end as telegraphsignal impulses.

5. In a telegraph system, a trunk circuit having a i'irst and a secondend, rst means in said trunk circuit comprising a telegraph receivingdevice responsive to changes in current conditions for receiving changesin current conditions through said second end as telegraph signalimpulses, second means in said circuit, comprising a potentiometerresponsive to said changes in current oonditions, for transmittingchanges in potential conditions through said first end as telegraphsignal impulses, to control a potential responsive telegraph signalreceiving device.

References Cited in the iile c1" this patent UNITED STATES PATEN'rsNumber Name Date 2,557,943 Clemens June 26, i951 FOREIGN PATENTS NumberCountry Date 533,371 Great Britain Feb. 12, 1941

